Solar energy has long been recognized as an alternative to traditional hydrocarbon energy sources, and the direct conversion of solar radiation to electricity via the photovoltaic process is a growing field for research and development. Currently marketed photovoltaic devices have, however, achieved only limited acceptance in specialized industries, such as expensive telecommunications installations, lighted buoys having low electrical consumption, and outer space applications.
In view of the significant costs of manufacturing single crystal photovoltaic cells, significant efforts have been ongoing for the past 20 years to develop a low cost polycrystalline photovoltaic cell. Tremendous financial investments have been and are continuing in the commercialization of polycrystalline and amorphous silicon solar cells, and also polycrystalline cadmium sulfide solar cells. Unfortunately, such cells inherently have a lower efficiency than single crystal cells, and the high capital investment and significant manufacturing costs for producing such cells according to current manufacturing technology have substantially inhibited market acceptance.
One promising approach to mass production of polycrystalline solar energy devices at a low cost utilizes a transparent vitreous substrate, such as common float glass, with photovoltaic films formed in successive layers on the substrate. In one "back wall" arrangement disclosed in U.S. Pat. No. 4,362,896, thin film layers of tin oxide, cadmium sulfide, cuprous sulfide, and an electrode material are applied to form the cell, with radiant energy passing through the glass and tin oxide layers before being absorbed in the CdS/Cu.sub.2 S heterojunction. In another similar polycrystalline photovoltaic cell, cadmium telluride is utilized in place of cuprous sulfide.
In order to achieve low cost manufacturing of such cells, the cadmium sulfide layer must be relatively thin, i.e., less than 10 microns. Also, it has been found that the formation of the cadmium sulfide layer by spray pyrolysis techniques, such as are disclosed in U.S. Pat. No. 4,338,078, are difficult to control. The varying temperatures of the glass substrate surface results in poor yield rates, which in turn significantly increases manufacturing costs.